OH a Peculiar Class of Optical Deceptions. [1831. 



but equal velocities. When looked at from a particular posi- 

 tion, they presented the appearance of a wheel with immove- 

 uble radii. 



When the two wheels of a gig or carriage in motion are 

 looked at from an oblique position, so that the line of sight 

 crosses the axle, the space through which the wheels overlap 

 appears to be divided into a number of fixed curved lines, 

 passing from the axle of one wheel to the axle of the other, in 

 general form and arrangement resembling the lines described by 

 iron filings between the opposite poles of a magnet. The effect 

 may be obtained at pleasure by cutting two equal wheels out 

 of white cardboard (Plate III. fig. 1), each having from twelve 

 to twenty or thirty radii, sticking them on a large needle two 

 or three inches apart, revolving them between the fingers, and 

 looking at them in the right direction against a dark or black 

 ground ; the greater the velocity of the wheels the more perfect 

 will be the appearance (fig, 2). 



When the dark-coloured wheel of a carriage is moving on a 

 good light-coloured road, so that the sun shines almost directly 

 on its broadside, and the wheel and its shadow are looked at 

 obliquely, so that the one overlaps the other in part, then, in 

 the overlapping part, luminous or light lines will be perceived 

 curved more or less, and conjoining the axle and its shadow, 

 if the wheel and shadow are superposed sufficiently; or, tending 

 to do so, if they are superposed only in part : the more rapid 

 the motion the more perfect is the appearance. The effect 

 may be easily observed by making a pasteboard wheel like one 

 of those just described, blackening it, sticking it on a pin, and 

 revolving it in the sunshine, or in candlelight, before a sheet 

 of white paper (fig. 3). If the wheel be converted into a 

 tetotum or top, by having a pin thrust through its centre, and 

 spun upon a sheet of white paper, the effect produced by the 

 wheel and its shadow will be obtained with facility, and in form 

 will resemble fig. 2. In all these cases no rims are required ; 

 the spokes or radii produce the effect. 



If a carriage wheel running rapidly before upright bars, as 

 a palisade or railing, be observed, the attention being fixed 

 upon the wheel, peculiar stationary lines will appear: those 

 perpendicular to the nave or axis will be straight, but the 

 others curved ; and the curve will be greatest in those which 



